1. Field of the Invention
The present invention relates to a method for recovering (meth)acrylonitrile, and more particularly, to a method for recovering (meth)acrylonitrile by separating acetonitrile and hydrogen cyanide, which are side products, from a reaction product obtainable from the production process for (meth)acrylonitrile through distillation in a distillation tower, in which method stable and efficient purification by distillation is performed over a long time by suppressing an increase in the differential pressure in the distillation tower.
As used herein, the term (meth)acrylonitrile refers to acrylonitrile or methacrylonitrile.
Priority is claimed on Japanese Patent Application No. 2009-093073, filed Apr. 7, 2009, the content of which is incorporated herein by reference.
2. Description of Related Art
Generally, in the production process for acrylonitrile or methacrylonitrile, an ammoxidation reaction is used.
In this production process for (meth)acrylonitrile based on ammoxidation, first, a hydrocarbon such as propylene or isobutene, ammonia and an oxygen-containing gas such as air are introduced into a reactor and are subjected to an ammoxidation reaction in the presence of a catalyst. If acrylonitrile is to be produced, propylene is used as the raw material hydrocarbon, and if methacrylonitrile is to be produced, isobutene is used as the raw material hydrocarbon. In this ammoxidation reaction, acetonitrile and hydrogen cyanide are generated as side products, together with the target product (meth)acrylonitrile. Thus, the ammoxidation reaction gas obtained from the reactor contains not only (meth)acrylonitrile but also acetonitrile and hydrogen cyanide, and also contains unreacted ammonia or other lightweight gases.
Therefore, first, the obtained ammoxidation reaction gas is sent to an ammonia absorption tower to add sulfuric acid thereto, and ammonia is removed from ammonium sulfate. The separated gas having ammonia removed in the ammonia absorption tower is subsequently sent to a (meth)acrylonitrile absorption tower, and (meth)acrylonitrile, hydrogen cyanide and the like are absorbed using absorption water that is supplied from the top of the tower. The obtained bottoms liquid is sent to a (meth)acrylonitrile recovery tower. In this (meth)acrylonitrile recovery tower, acetonitrile is separated to recover (meth)acrylonitrile, and the (meth)acrylonitrile obtained from the (meth)acrylonitrile recovery tower is further purified in a purification tower, to thereby obtain (meth)acrylonitrile as a product.
There have been hitherto suggested various suggestions for the purpose of enhancing the recovery efficiency for (meth)acrylonitrile at the (meth)acrylonitrile recovery tower in this production process for (meth)acrylonitrile. For example, JP-T-2002-518353 suggests a method of separating by distillation the overhead stream enriched with acrylonitrile at the recovery tower, a side stream composed of water with less foreign materials, and a bottom stream containing organic impurities.
JP-A-55-104243 also suggests a method of separating by distillation a distillate fraction containing (meth)acrylonitrile, hydrogen cyanide and a small amount of water from the overhead of a distillation tower, acetonitrile from a middle tray, and water from the bottom, respectively.